FIG. 10 illustrates a general view of a coke drum A. A drum body 1 has a cylindrical shape. A head plate 2 is attached to the top of the drum body 1. An inverted-cone-shaped bottom plate 3 is formed at the bottom of the drum body 1. A cylindrical skirt 4 is attached around the boundary between the drum body 1 and the bottom plate 3. This skirt 4 is a support member for the coke drum A, and is configured to be fixed onto a concrete foundation 5 with bolts or the like.
The coke drum, which is a cylindrical vessel, has the following features in particular.
1) Thin and large in diameter
A pyrolytic reaction that occurs in the coke drum does not require a high pressure inside the vessel. A consecutive pyrolytic reaction is caused by putting in heated residual oil (the design pressure of the coke drum: 0.5 MPa [approximately 5 atm]). Because of a low design pressure, the coke drum may be reduced in plate thickness to result in a pressure vessel thin-walled and large in diameter. For other pressure vessels such as reactors, where a chemical reaction is caused by causing the internal pressure of the vessel to be high, the design pressure is as high as approximately 1 MPa to approximately 10 MPa (approximately 10 atm to approximately 100 atm).
2) Repetitive thermal cycle between approximately 100° C. and approximately 500° C. (cycle time: 12 hours to 24 hours)
There is no concept of “metal fatigue” due to thermal cycle repeated loading for common pressure vessels, which are maintained in a certain high-temperature state once operation is started.
On the other hand, the coke drum is a unique vessel repeatedly subjected to a thermal cycle of approximately 100° C. to approximately 500° C. to approximately 100° C. in an extremely short cycle of 12 hours to 24 hours in its regular operation. Therefore, the drum repeats expansion and contraction during operations, so that there is a problem in that the attachment part of a skirt is subjected to the load of thermal stress of extremely large amplitude, and is likely to be damaged by “metal fatigue.”
3) Damage due to metal fatigue becoming apparent
The concept of “metal fatigue” due to thermal cycle repeated loading is unique to the coke drum, which operates with varying temperature in a short period of time, among the pressure vessels.
4) Increase in fatigue damage due to shortened cycle time
Users have been trying to reduce operating cycle time in order to make profits from producing more light oil and coke through refining in a shorter period of time. A shortened operating cycle results in reduction in heating and cooling time, thus causing sharp changes over time in the temperature distribution near the attachment part of the skirt. This leads to “generation of a greater thermal stress,” thus increasing fatigue damage.
5) No establishment of a design method that considers metal fatigue as a limit state
It is necessary from Item 2) above to design a coke drum in consideration of “metal fatigue” due to thermal cycle repeated loading, but such a designing method is not yet established. As a result of not taking metal fatigue due to a thermal cycle into consideration in designing, the occurrence of fatigue damage have been reported in many cases. Like for other pressure vessels, however, designing considers only static temperature and pressure, a dead load, a seismic load, a wind load, etc.
6) Extremely heavy operating-time dead load of 2000 tons to 3000 tons
The operating-time dead load is extremely heavy because of residual oil and water put inside.
As described above, there are circumstances that are unique to the coke drum and are not shared by common pressure vessels. A typical conventional skirt support structure is as illustrated in FIG. 11. A curved thick steel plate is formed from the vertical drum body 1 to the sloped bottom plate 3. The upper end of the skirt 4 is welded to the neighborhood of the upper end of the bottom plate 3 (that is, the boundary with the drum body 1). Reference sign 6 denotes the weld.
As described above, the coke drum is subjected to repeated heating and cooling. As illustrated in FIG. 12, the coke drum bulges outward near the joint part above the skirt 4, but does not move below the skirt 4 because the temperature does not increase (does not become high), so that high stress is generated in the joint part (see the drawing of (A)). On the other hand, when the temperature decreases at a cooling time, the coke drum tries to return inward above the skirt 4, but also tries to keep the high-temperature state below the skirt 4, so that a deformation opposite to that of the drawing (A) remains (see the drawing of (B)). According to the conventional art, by repeating such expansion and contraction, cracks are likely to be caused at the upper end of the attachment part of the skirt 4, that is, near the weld, as indicated by sign C in FIG. 11.
Therefore, the attachment part of the skirt has a short useful service life, and may suffer from generation of cracks as early as in about ten years.
Further, the conventional art of FIG. 11, which performs joining only by welding, thus making it important to control the quality of the weld, has a disadvantage in that the durability depends on quality including the presence or absence of a welding defect and the finished state of welding.
Patent Documents 1 and 2 illustrate conventional art for support structures of coke drums.
The coke drum of Patent Document 1 has an annular jacket formed around where a skirt is welded to a drum. Cooling fluid is caused to flow through the jacket during a quenching process during operations to reduce metal stress around the weld.
The coke drum of Patent Document 2 supports the bottom part of a drum vessel using a support element that provides a large contact surface. The support element has a bearing portion that tapers inward beneath a knuckle that separates from the sloped lower section of the drum vessel. The bearing portion is a funnel-shaped member that extends along the sloped surface of the drum vessel, and has a large contact surface. The support element has a narrow lower portion fixed onto a foundation with bolts.
However, the conventional art of Patent Document 1, which makes it necessary to cause cooling fluid to flow through the jacket at the time of a quenching process, has a disadvantage in that running costs are necessary.
Further, according to the conventional art of Patent Document 2, it is difficult to ensure such manufacturing accuracy as to cause the drum vessel and the support element to be in surface contact. In practice, the support element does not come into surface contact but only comes into point or liner contact with the bottom part of the drum vessel. The contact pressure is high where contact is made with a narrow area as in this case. Thus, there is a disadvantage in that deformation or distortion is likely to occur, so that there has been no case of its practical use.